This paper deals with the realisation of the prospective participle, the most common way to express the future in Basque. The authors start from present-day variation, as synthesised by the Royal Academy of the Basque Language: the adjunction of the suffix -ko to the perfective participle, except for the forms ending in -n or -l, which in Eastern dialects take the genitive suffix -en. Based on the study of both old texts from different dialects and works on verbal morphology, the authors show that the variability of the marking of the prospective participle is greater than usually thought: on the one hand, Labourdin texts attest for a late use of the suffix -(r)en with vowel-ending participles (e.g. biziren, ikusiren, harturen); on the other hand, in the 17th century forms such as izaren 'izanen' and egoren 'egonen' arise in Labourdin and Western Low-Navarrese dialects. The authors consider two possible explanations for the prospective forms of the type izaren: 1) in phonological terms, a dissimilation of nasality (cf. Michelena 1990); and 2) in syntactical terms, a change in the selection of the complement argument by the T-head, lexicalised by the genitive suffix -(r)en. In this view, the forms such as emaren or egoren are the result of adding the suffix -(r)en not to a perfective participle eman 'given' or egon 'rested' (as in ikusi-ren 'seen-PROSP'), but to a verbal root -ma- or -go-, respectively.